Moth-Inspired Coating is Perfectly Nonreflective

Photonics.comMay 2010
FREIBURG, Germany, May 7, 2010 — A new hybrid nanocoating that ensures a perfectly nonreflecting view on displays and other transparent surfaces has recently been unveiled. The nanocoating is applied during manufacturing, which removes a step in the process and reduces cost. The coating is also scratchproof and easy to clean.

It was actually inspired by moths because, unlike other insects, a moth’s eyes are perfectly nonreflecting, which helps the insects hide from predators as they search for food at dusk. They have tiny protuberances smaller than the wavelength of light that form a periodic structure on the surface. This nanostructure creates a gentle transition between the refractive indices of the air and the cornea. As a result, the reflection of light is reduced and the moth remains undetected.

Research scientists at the Fraunhofer Institute for Mechanics of Materials IWM have adopted this artifice and adapted it to a range of different applications. On eyeglasses, cell phone displays, fitting or panel covers, transparent surfaces are generally useful only if they allow viewing without light reflecting back. Whereas conventional methods apply the antireflection coating in a separate step after production, the Fraunhofer scientists have found a way of reducing light reflection during actual manufacture of the part or component.
This car speedometer cover with two antireflective circular surface areas was manufactured in just one step. (Image: Fraunhofer IWM)
“We have modified conventional injection molding in such a way that the desired nanostructure is imparted to the surface during the process,” said Dr. Frank Burmeister, project manager at the IWM.

For this the researchers have developed a hard material coating which reproduces the optically effective surface structure. “We use this to coat the molding tools. When the viscous polymer melt is injected into the mold, the nanostructures are transferred directly to the component,” he said.

Because no second process step is required, manufacturers achieve an enormous cost saving and also increase efficiency. “Normally the component would have to undergo an additional separate process to apply the antireflex coating,” Burmeister added.

Normal plexiglass and some antireflex coatings are particularly sensitive, but the scientists are producing wipe-resistant and scratchproof surfaces. For this purpose the injection mold is additionally flooded with an ultrathin organic substance made of polyurethane. The substance runs into every crevice and hardens, like a two-component adhesive. The result is an extremely thin nanocoating of polyurethane on which the optically effective surface structures, which are just one ten-thousandth of a millimeter thick, are also reproduced.

Working in cooperation with industrial partners, the research scientists now aim to develop components for the auto industry, for example, which are not only attractive to look at but also durable and easy to clean.